Repairing Chimney Flashing

Technical Procedures Disclaimer

Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.


We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.



A. This procedure includes guidance on repairing chimney flashing that is missing, deteriorated or damaged.

B. Flashing is an integral part of a roofing system and protects against water infiltration at roof hips and valleys; penetrations such as chimneys; where roofing meets a vertical surface such as a parapet or at a porch roof; or at drip edges.

C. Copper, with copper nails, galvanized sheet metal with hot-dipped galvanized nails, or terne-coated stainless steel with stainless steel nails are the primary materials used for flashing.


D. Safety Precautions:

  1. Wear rubber-soled shoes that have non-slip or grid type tread (preferably high top sneakers for good ankle support. Avoid wearing loose clothing.
  2. Wear a safety belt or harness secured to a substantial chimney or to a window on the opposite side of the house. Leave only enough slack so you can work comfortably in one area, and adjust the slack as you work on other sections of the roof.
  3. Be sure the roof is clear of debris and water.
  4. Do not work on wet or snow covered roofs, or on a brittle roof surface. Work on cleated walkboards.
  5. On steep roofs:
    • Secure chicken ladders or cleats at the top for adequate footing.
    • Safety lines should be tied and secured with manila rope.
    • Carry a limited amount of materials so that balance and footing are not impaired.
    • Provide scaffolding, ladders and working platforms as required to execute this work.

D. Historic Structure Precautions: Examine, measure, and record existing flashing application methods at eaves and gable edges, ridges, chimneys, parapet vents, and other roof penetrations. Take note of reglet details and methods of fastening sheets together.

E. See 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:

  1. Safety Precautions
  2. Historic Structures Precautions
  3. Submittals
  4. Quality Assurance
  5. Delivery, Storage and Handling
  6. Project/Site Conditions
  7. Sequencing and Scheduling
  8. General Protection (Surface and Surrounding)

    These guidelines should be reviewed before performing this procedure and should be followed, when applicable, along with recommendations from the Regional Historic Preservation Officer (RHPO) or designated representative.


A. Flashing is functioning as it was intended when there are no cracks, splits or torn areas. It should not show any signs of corrosion. Where flashing meets a vertical surface, all cap flashing should be secure and the flashing itself should be soundly anchored.


A. Check flashings as a part of the annual roof inspection (preferably in late autumn). Repair defects immediately because flashing is a critical part of the roof fabric. Small leaks in the flashing which are neglected may lead to major faults in the roof system.



A. Flashing material: 16 oz. or 20 oz. sheet copper; lead- coated copper; 26 gauge galvanized steel. Metal should match remaining metals on the roof.

B. Inorganic Zinc-Rich Primer: Zinc dust - zinc oxide primer by 3M, Mobil, or Tnemic to paint galvanized flashing.

C. Sealant: Clear Silicone Rubber Sealant as manufactured by Dow Corning, General Electric, or equivalent.

D. Nails used for fastening copper shall be copper or hardware bronze of stronghold type or equal, with large flat head. They shall not be smaller than No. 12 stubs gauge (0.109 inches) and of sufficient length to penetrate roofing boarding. For galvanized metal use hot dipped galvanized nails.

E. Building Paper: For copper, use red rosin paper having minimum weight of 2 pounds per square. For galvanized metal, roofing felts may be used.

F. Mortar as directed by RHPO.


A. Safety belt or harness; protective gloves and gear.

B. Chicken ladder; ladders and scaffolding.

C. Straight snips for cutting straight or curved lines in sheet metal 24 gauge or lighter.

D. Handy tongs for bending the edges of the solder.

E. Hawk, pointing trowel



A. Outside, inspect flashing for splits, holes or corrosion.

B. Inside, inspect the underside of the roof deck for evidence such as water stains or damp wood; inspect the ceiling and walls around fireplaces and chimney flues for stained or spalling plaster or wallpaper, or peeling paint.


A. Surface Preparation:

  1. Examine, measure, and record existing flashing and its method of application. Take note of reglet details and methods of fastening sheets together.
  2. Select sheetmetal type to be used making sure it is compatible with the remaining flashing on the roof, the gutters and the downspouts.
  3. Cutting and Bending:
    • Mark all bends, cuts, and notches which will be required using existing pieces as patterns. Regular metal cutting shears can cut 16-oz. copper, or 26 gauge galvanized steel.
    • To aid in forming the bends on the base and counterflashing aprons, clamp a 2 x 4 over the flashing piece, with the bending line at the edge of the worktable. Use an additional piece of 2 x 4 as blocking to place against flashing and bend it into a 90o bend by striking it with a hammer. To create a hem, unclamp and continue hammering against a 2 x 4 to bend metal edge over again.
    • Cut base flashing pipes so that they will extend under the shingles a minimum of 4", and up the chimney face a minimum of 4". The total length of each piece will depend on the exposure of the shingles.
    • Counterflashing pieces must overlap the base flashing by a minimum of 4". At corners there should be a 2" for double overlap.
  4. Rosin paper should be laid on the roof decking beneath the base flashing if none exists. Use only copper nails with copper flashing, or hot-dipped galvanized nails if using galvanized sheet metal. The nails should be sized to allow for at least 1" penetration into roof deck.
  5. The reglet is the slot cut into the chimney mortar joints into which the cap flashing is set. It goes straight across the front and is stepped along the two sides. If there is not a cricket at the back of the chimney the reglet slot is also straight across.
    • Cut the slot by hand using a cold chisel. Experienced operators may also use a diamond blade with a water spray attachment in a hand- held circular saw, a portable grinder, or a circular saw with a carbide masonry blade.
    • Cut the top and bottom portions of the joint to a depth of 1-1/2" being careful not to cut the brick.
    • Use a cold chisel to knock out mortar between cuts.


A. Apron Base Flashing:

  1. Remove shingles on the back and sides of the chimney only. Remove only as far as the next full shingle beyond the 4" minimum. Save the shingles for reinstallation.
  2. Install apron flashing over shingles on lower (front) slope with a hem at the bottom edge.
  3. Place one nail at each top corner where it will be covered by the first piece of side base flashing.

B. Stepped Base Flashing:

  1. Lay the first piece of base flashing so that its bottom edge covers the nail of the apron flashing.
  2. Nail the base piece at its upper edge, close to the chimney. If the roofing material is barrel tile, bend the free edge of the base piece up about 1/2" to act as a trough to direct water down the slope of the roof.
  3. Reinstall the first shingle. Nail so the bottom edge of the shingle is 1/2" below the bottom edge of the base flashing and the top edge of the shingle is 2" below the top edge of the base flashing. Make sure the lap of the reinstalled shingle matches the lap of the remaining roof.
  4. Continue to interweave base flashing, and shingles until both sides are complete, making sure to match exposure of the remainder of the roof.
  5. At the rear of the chimney, install a continuous strip of base flashing whose ends wrap each corner by 2".
  6. Relay shingles over this piece of base flashing making sure the nails of each shingle are covered by the lap of the shingle above. The lap should match the remainder of the roof.

C. Apron Counterflashing:

  1. For each piece of counter flashing, bend a 1/2" lip on the edge that slips into the reglet.
  2. Slide the apron counter flashing into the reglet, overlapping the corners about 4". A hem can be bent into this piece as well to provide extra stiffening.
  3. Overlap the base flashing a minimum of 4". It can however, extend all the way down the vertical surface.
  4. Set rolled metal wedges, no more than 12" apart, to hold cap flashing into the reglet. Use lead or copper if flashing is copper. Use lead if flashing is galvanized.

D. Stepped Counterflashing:

  1. Insert pieces of counter flashing, overlapping each succeeding piece 2". Trim the bottom edge of each piece to follow the slope on the roof.
  2. Each piece should step up above the previous piece a maximum of three courses of brick.
  3. Overlap base flashing by at least 4", or allow it to come all the way down the vertical wall if desired.
  4. Insert rolled metal wedges to hold the counter flashing tight. Insert a minimum of two wedges per piece of counter flashing, no more then 12" on center.
  5. The final piece of cap flashing, at the rear of the chimney, should include a minimum 2" return at each end.

E. Repoint the reglet with a mortar that matches the original in composition, color, and shape of joint. Caulk can be also used, but this will require annual inspection and maintenance.

F. If it is not possible to cut a reglet in the wall or chimney, secure each piece of counter flashing with brass screws driven into lead anchors. Flare the top of the flashing 1/4" to receive a bead of high quality, long lasting, UV resistant exterior caulk. This solution will require annual inspections and periodic replacement of the caulk seal.